Various development efforts to have porous separation membrane having a good separation property and being capable of allowing hot water to permeate have been made, and a polysulfone porous membrane has especially received much attention. The invention of a thermal resistant polysulfone porous membrane using polysulfone according to the general formula of (1) and polyethersulfone according to the general formula of (2) are described in Japanese Laid-Open Patent Publication Nos. Sho63-99325 and Hei5-137982. ##STR1##
Among the above-mentioned inventions, the polysulfone porous membrane using the polysulfone of the formula (1) has a fairly good permeability, but it has a drawback wherein the membrane ruptures or becomes unsuitable to be used as a separation membrane due to its morphological change when it is subjected to a treatment of moist heat under a high temperature and a high pressure.
In addition, although the polyethersulfone porous membrane using the polyethersulfone of the formula (2) is characterized by having a relatively good re-percolation characteristic and resistance to moist heat, its performance is not sufficient as a ultra filtration membrane or as a precision filtration membrane because its water flux is extremely low by being at not more than 2 L/m.sup.2.hr.mmHg.
An invention directed to a polyallylsulfone porous membrane having a chemical formula of (3) is described in Japanese Laid-Open Patent Publication No. Hei1-184001. This polyarylsulfone porous membrane has an extremely high resistance to moist heat, however, it has a drawback that its water flux is low at not more than 2 L/m.sup.2.hr.mmHg and it has a low hydrophilicity. ##STR2##
The inventions directed to a porous membrane comprising a mixture of polysulfone and polyethersulfone are described in Japanese Laid-Open Patent Publication Nos. Sho54-144456 and Sho62-221402. The polysulfone porous membrane of this type has a relatively high water flux, but has a drawback that its resistance to moist heat is not sufficient and its water flux decreases rapidly when a hot water having a temperature not less than 80.degree. C. is percolated for an extended period of time, and while its mechanical strength is not sufficient and its ease of handling is not satisfactory.
In addition, in case of a porous membrane obtained from materials of polysulfone and polyethersulfone, because the material itself is hydrophobic, during the manufacture of a membrane by a wet forming method, a process of imparting hydrophilicity to the membrane has been employed, wherein pores are formed by a solidification process while adding a large quantity of hydrophilic additives such as polyethylene glycol and polyvinylpyrrolidone to the dope, or then after these hydrophilic additives are cross-linked, or a hydrophilic polymer is coated on the membrane surface after the formation of pores followed by maintaining the hydrophilic polymer on the membrane surface.
Generally, to provide a sufficient hydrophilicity to the membrane, it is necessary to have the equilibrium rate of moisture content of the membrane at more than 2% by weight by employing the above mentioned treatment.
There was a problem where the hydrophilic additive or the hydrophilic polymer of these types dissolves during the use of the membrane from the membrane thus obtained by these manufacturing methods, because a large quantity of hydrophilic additives or hydrophilic polymer and the like are retained inside of the membrane or on the surface thereof.
Furthermore, in case of a hollow fiber membrane obtained by the conventional wet forming methods to increase the water flux, it was necessary to reduce the membrane thickness or to increase the porosity of the membrane. However, when such operation is employed, a drawback may be encountered wherein the ease of handling of the membrane, and thus the ease of handling during a production process, decreases because the mechanical properties (breaking strength and breaking extension) decrease.